1. Field of the Invention
The present invention relates to a region detecting method and a region detecting apparatus for detecting a predetermined region to be extracted from a purposed image, particularly to a region detecting method and a region detecting apparatus for extracting the face of a person from a photographic image or the like.
2. Description of the Related Art
In recent years, an image processing method is proposed which automatically extracts a specific region from a photographic image. Particularly to extract a face region of a person, the method is widely used to count personal images. As an image processing technique for these personal images is developed, it is requested not only to perform accurate recognition but also to quickly retrieve the face region and moreover identify a person.
As the above-mentioned face-region detecting method, the following methods are proposed: a method for using a threshold for determining the face region from a purposed image and detecting the face region in accordance with the threshold and a method for deciding the region of a face by using a neural network and thereby extracting a skin color.
However, the method for using the threshold of a skin color for determining the face region from a purposed image and detecting the face region in accordance with the threshold has the following problems. Although high-speed retrieval can be made, it is difficult to decide a threshold, when a skin color is included in background colors. Not only erroneous recognition occurs but also the recognition rate depends on the race of the individual. Thus, erroneous recognition occurs depending on the photographing conditions for an object, and the recognition rate can be lowered.
However, the method for deciding the region of a face by using a neural network and thereby extracting the skin color has a problem that a lot of time is required for recognition when attaching importance to accuracy but the recognition rate extremely lowers when attaching importance to speed because there is a trade-off relation between improvement of the recognition rate and time required for retrieval.
Thus, the inventor of this application proposed a method for detecting a lip and then starting retrieval of the position of a face when the skin color around the lip can be confirmed by paying attention to a lip whose color information does not greatly depend on a race for extraction of a face and using the feature that a skin color is present around the lip in “A Detection Method of Face Region in Color Images by Using the Lip Detection Neural Network and the Skin Distinction Neural Network” in The Transactions of The Institute of Electrical Engineers of Japan, 2001, Vol. 121-C, No. 1, pp. 112-117. However, because the above method is a method for determining a lip/skin color one pixel by one pixel by using a neural network, the method also has problems that it takes a lot of time to obtain a high recognition rate and it is difficult to apply the method to a dynamic image or practically use the method.